Facile synthesis of a novel structured Li[Ni0.66Co0.1Mn0.24]O2 cathode material with improved cycle life and thermal stability via ion diffusion
Introduction
Li-ion battery (LIB) has become main power sources for portable electronic devices due to its high energy density and power capability since its introduction by Sony in 1991 [1], [2], [3], [4]. By virtue of comparatively low cost, low toxicity, large reversible capacity and especially high energy density, Ni-rich Li[Ni1-xMx]O2 (M = metals) materials have attracted much interest and are considered as one of the most promising cathode materials for hybrid electric vehicles (HEVs) and electric vehicles (EVs) [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15]. Unfortunately, Ni-rich layered materials suffer from poor thermal stability due to oxygen release from the highly delithiated Li[Ni1-xMx]O2 host structure, which may lead to a severe thermal runaway with explosion [16], [17]. Cycle performance of Ni-rich Li[Ni1-xMx]O2 materials is also unsatisfactory because Ni4+ in delithiated Li[Ni1-xMx]O2 materials is unstable and can be reduced to NiO. In addition, Ni4+ in delithiated Li[Ni1-xMx]O2 materials possess high oxidizability and can react with electrolyte [18].
One of the conventional methods to improve the thermal stability and cycle performance of Ni-rich Li[Ni1-xMx]O2 cathode materials is coating nano-scaled materials (such as metal phosphate AlPO4, Li1.3Al0.3Ti1.7(PO4)3 and Li3PO4, metal fluoride AlF3) on their surfaces [19], [20], [21], [22], [23]. However, this kind of nano-coating is incomplete, and part of the cathode particle surface is still exposed to HF in the electrolyte, causing electrochemical and thermal performance degradation of cathode materials [24].
Recently, Yang-Kook Sun et al. constructed micro-scale core-shell and concentration-gradient Ni-rich Li[Ni1-xMx]O2 materials to overcome the problem of incomplete nano-scale coating [24], [25], [26]. It is found that core-shell Ni-rich Li[Ni1-xMx]O2 materials present better thermal stability and cycle performance than normal structured materials [27], [28]. However, owing to the structural mismatch and the difference in volume change between the core and the shell, a large void forms at the core/shell interface after long-term cycling, leading to a sudden drop in capacity. Compared with core-shell materials, structural mismatch and the difference in volume change between the core and the shell are significantly reduced in concentration-gradient materials [29], [30]. However, the synthesis procedures of concentration-gradient materials are complicated and difficult to control, leading to poor product consistency.
Ion diffusion phenomenon of transition metals are often observed and difficult to avoid in core-shell and concentration-gradient Li[Ni1-xMx]O2 materials during high temperature calcination. In this work, a novel structured Li[Ni0.66Co0.1Mn0.24]O2 with the combined characteristics of core-shell and concentration-gradient structures is facilely synthesized from core-shell precursor via ion diffusion during high temperature calcination. This novel structure is expected to avoid structural mismatch and the difference in volume change between the core and the shell. Moreover, this facile synthesis procedure can be easily controlled, which is suitable for industrial application. Detailed comparative investigations between novel structured and normal Li[Ni0.66Co0.1Mn0.24]O2 are performed.
Section snippets
Preparation of precursors and layered oxides
Precursors [(Ni0.8Co0.1Mn0.1)0.6(Ni0.45Co0.1Mn0.45)0.4](OH)2, [Ni0.66Co0.1Mn0.24](OH)2, [Ni0.8Co0.1Mn0.1](OH)2 and [Ni0.45Co0.1Mn0.45](OH)2 were synthesized by a co-precipitation method.
To synthesize the designed core-shell precursors [(Ni0.8Co0.1Mn0.1)0.6(Ni0.45Co0.1Mn0.45)0.4](OH)2, 30 L aqueous solution containing 60 mol NiSO4·6H2O, 7.5 mol CoSO4·7H2O and 7.5 mol MnSO4·H2O (cationic ratio of NiCoMn = 8:1:1) was pumped into a continuously stirred tank reactor (CSTR, capacity of 170 L) under N2
Results and discussion
Fig. 1 shows the SEM images of normal, core and core-shell precursors. All the precursors exihibit spherical structure assembled from closely packed small primary grains. However, the core-shell precursors display a varied primary grains for core and shell surface. Clearly, the core has a needle shaped primary grains, while the shell presents polyhedral primary particles. The variation of primary grains form core to shell is possibly related to the change of Ni/Co/Mn ratio during reaction. ICP
Conclusion
Core-shell precursors [(Ni0.8Co0.1Mn0.1)0.6(Ni0.45Co0.1Mn0.45)0.4](OH)2 with average composition of [Ni0.66Co0.1Mn0.24](OH)2 is designed and prepared via a co-precipitation route. Interestingly, a novel structured Ni-rich layered oxide cathode LiNi0.66Co0.1Mn0.24O2 that consists of inner high-capacity core, concentration-gradient layer and outer stable shell is facilely synthesized from the as-prepared core-shell precursors after calcined at high temperature of 870 °C. Correspondingly, the
Acknowledgements
This work was financially supported partly by NSFC (51272175, 21301127, 21503148) and Tianjin Sci. & Tech. Program (15JCTPJC58000).
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